Clinical analyzers, be they large, high throughput machines, or smaller, lower throughput machines used in doctor's offices, commonly feature dried test elements for assaying liquids, from cartridges of such test elements. Representative cartridges are shown in U.S. Pat. No. Re. 30,595, especially FIG. 5. Such cartridges are automatically assembled by machine, so that there is little concern for how the test elements are fed into a stack in the cartridge. That is, their proper orientation is assured by quality control maintained by the automated process. Furthermore, many test elements are symmetric in nature in their top surface, that is, have an aperture for deposit of the sample liquid that is symmetrically located on that top surface. Therefore, such elements are somewhat indifferent as to their orientation. However, those shown in, e.g., U.S. Pat. No. 4,273,639, are asymmetric since the two liquid deposit apertures are located at one end only, and spaced away from the longitudinal center line of the test element. These test elements, often used for potentiometric measurements, are very sensitive to proper orientation when they are assembled in the cartridge, for readily apparent reasons. Backwards insertion will, of course, locate the liquid deposit apertures at the wrong end. Upside-down insertion is Just as unsatisfactory, since in that case the surface to receive the liquid is no longer the top surface, but underneath. (Each of the elements described in the aforesaid two patents is asymmetric about the center plane that extends parallel to the plane of the element.) Nevertheless, as long as automated equipment assembles such test elements into cartridges, the risk of improper orientation is minimized.
Some analyzers, however, have the operator assemble the test elements into a cartridge by hand, prior to placing the cartridge in the analyzer. Particularly, this is desired if the tests to be run are unique for each patient sample. In such a case, it is not possible to prepackage a variety of different test elements in all the possible combinations that a doctor may wish to test. In these instances of hand loading, the risks of improper orientation rise substantially, particularly if one or more of the test elements happens to be asymmetric as described above.
There has been a need, therefore, prior to this invention, to have a cartridge for test elements that will accept such test elements only in their proper orientation.